【作者】 潘恩宝；

【导师】 邸洪双；

【作者基本信息】 东北大学 ， 材料加工工程， 2017， 博士

【摘要】 为适应新能源、轻量化、长生命周期的汽车产品发展需要,提升钢铁产品竞争力,亟待开发高品质汽车用钢。其中,冷轧钢进行退火后被广泛应用于汽车上,特别是以冷轧双相钢、热镀锌双相钢等冷轧钢为代表。如何对冷轧双相和和热镀锌双相钢的退火工艺进行优化,改善钢板表面质量、提高双相钢的强度和塑性之间的匹配还有待深入研究;另外对镀层的质量控制,如镀层的剥落机制、抗粉化性能也需要开展进一步的工作。本文以Fe-0.15C-1.78Mn-0.1Si-0.08Ti成分双相钢为实验材料利用热模拟实验机研究了其连续冷却转变行为。结合显微组织分析,建立了实验钢的连续冷却转变曲线;利用带钢连续退火模拟实验机研究了冷轧双相钢和热镀锌双相钢的连续退火工艺、组织和性能之间关系,探讨了连续退火工艺对实验钢组织与性能的影响规律及其机理。以DC51D+ZF钢和SCG270钢为实验材料,系统研究了合金化镀层的剥落机制、抗粉化性能和相结构。本文的主要工作及结果如下:1.获得了 Fe-0.15C-1.78Mn-0.ISi-0.08Ti实验钢的连续冷却转变曲线,明确了不同连续冷却转变条件下的微观组织演变规律,并获得了实验钢的临界冷却转变温度Ac1(720℃)和Ac3(870℃)以及获得马氏体的最小临界冷却速率(>5℃/s)。该结果可为冷轧双相钢和热镀锌双相钢连续退火工艺参数的制订提供理论依据。2.明确了退火温度、保温时间、冷却速率对热镀锌双相钢连续退火后组织与性能的影响规律。(i)当退火温度较低时(如790℃或820℃),马氏体主要分布在铁素体晶粒内部,且马氏体岛多呈带状分布;当退火温度增至850℃时,促进了孪晶马氏体的形成,且带状组织分布不明显,同时马氏体的体积分数增加(约为19.6%)。综合分析,当退火温度为850℃时,实验钢具有良好的综合性能:Rm=840MPa;A=18%;Rp0.2/Rm=0.55。(ii)在一定的退火温度(850℃)条件下,随着退火时间的延长,铁素体的平均晶粒尺寸变大,马氏体的体积分数先增加后减少;当退火时间为100s时,马氏体的含量较多,且分布较为均匀。随着退火时间的延长,实验钢的抗拉强度和屈服强度均呈现先上升后下降的趋势,但延伸率的变化较小。(iii)通过改变冷却速率,可以对铁素体和马氏体的体积分数进行有效控制,获得低屈服、高强度的热镀锌双相钢。在850℃退火后,随着冷却速率的变化,抗拉强度略有增加,这主要归因于铁素体、马氏体及两者之间的交互作用;随着冷却速率的增加,实验钢的屈服强度呈先上升后下降的趋势。3.阐明了加热速率、加热温度及缓冷温度等工艺参数对冷轧双相钢连续退火过程中组织演变的影响规律。(i)在一定的退火温度(810℃)条件下,随着加热速率的提高,马氏体岛的体积分数增加,其形貌由长条状变为等轴状分布,其平均尺寸减小;当加热速率为70℃/s时,马氏体岛的分布更为细小、弥散,且尺寸接近1μm。随着加热速率的提高,实验钢的屈服和抗拉强度均有不同程度的改善,但延伸速率略有下降;当采用较高的加热速率(如70℃/s)时,实验钢的抗拉强度级别可达920 MPa,且综合性能良好,例如强塑积达16.4 GPa·%。(ii)当加热温度从780℃增至810℃时,实验钢中马氏体的体积分数由29.5%增至37%,且马氏体板条的尺寸增加,晶界由清晰变得模糊;随着进一步提高加热温度,马氏体的相分数变化趋势有所减缓。随着加热温度的升高,实验钢的抗拉强度和延伸率均呈现先增大后减小趋势,而屈服强度有所增加。(iii)随着缓冷温度的降低,生成的取向附生铁素体的体积分数增多,马氏体的体积分数减少,取向附生铁素体的固溶碳含量较低,使实验钢兼具较低的屈服强度和良好的塑性。4.明确了合金化镀锌板基板表面粗糙度、晶体学织构和镀层相结构对合金化镀层性能的影响。(i)针对DC51D+ZF基板表面晶体学织构研究表明,基板表面的{001}-{101}取向更有利于合金化镀层中δ相的生成,基板表面的{111}-{113}-{313}更容使镀层中形成r相。合金化镀层的抗粉化性能随着镀层中Fe含量和r相层厚度的增加而减弱,然而其剥离性能却随着r相层厚度增加得到了改善。锌层本身的织构和基板的成形能力也是影响镀层抗粉化性能的原因。(ii)针对两个厚度(0.7 mm/1.6 mm)SCG270合金化镀锌板不同的抗粉化性能进行分析,结果表明,镀层的Fe含量不同,导致了镀层不同的相结构;基板的粗糙度的不同,合金化程度不同,厚基板的粗糙度要大于薄基板的粗糙度,粗糙度大,其合金化程度加快,最后导致了镀层中Fe含量的不同;随着基板厚度的增加合金化镀层的厚度也增加,而更厚的镀层其抗粉化能力越差;镀层厚度的不均匀性也是导致镀层粉化性不同的因素。更多还原

【Abstract】 To meet the needs of new energy,lightweight,long life cycle of automobile products,and to enhance the competitiveness of steel products,the development of high quality automobile steels should be accelerated.Cold-rolled steels after annealing are widely used in automobiles,especially the cold-rolled dual-phase steels and the hot-dipped galvanizing dual-phase steels.However,how to optimize the annealing processes of the dual-phase steels to improve the surface qualities and to promote the matching between the strength and plasticity of steel plates needs a further study.Additionally,to control the coating quality,such as the peeling mechanism of coating and anti-chalking performance,some other work is also needed.In this dissertation,the continuous cooling transformation of dual-phase steel was studied by thermo-mechanical simulation experiment,and the continuous cooling transformation curve was also established based on the microstructural analysis.The relationships among the continuous annealing process,microstructures and properties of the cold-rolled dual-phase steels and the hot-dip galvanizing dual-phase steels were studied with the continuous annealing simulator.The effects of the continuous annealing process on the structures and properties of the experimental steels and the corresponding mechanisms were discussed.The peeling mechanism of coating,anti-chalking performance and the phase structure of the automobile steel coating were systematically studied.The main work and results of this dissertation are as follows:1.The continuous cooling curve of the Fe-0.15C-1.78Mn-0.1Si-0.08Ti steel was measured,and the microstructural evolution laws under different continuous cooling conditions were specified.Furthermore,the critical cooling transformation temperatures Ac1(720℃)and Ac3(870℃),and the minimum critical cooling rate of martensite(>5℃/s)were obtained.These results can provide theoretical basis for the continuous annealing processes of cold-rolled dual-phase steels and the hot-dip galvanizing dual-phase steels.2.The effects of annealing temperature,holding time and cooling rate on the microstructures and properties of hot-dip galvanizing dual-phase steel were studied.(i)When the annealing temperature is low(e.g.790℃C or 820℃),the martensite is mainly distributed in the grains,and the martensite islands mainly reveal banded microstructure distribution.A further increase of the annealing temperature to 850℃ facilitates the formation of twin martensite,and the banded microstructure distribution becomes less appreciable.Meanwhile,the volume fraction of the martensite is increased(about 19.6%).With a comprehensive analysis,the experimental steel has favorable combination properties when the annealing temperature is 850℃:Rm=840 MPa;A=18%;Rp0.2/Rm=0.55.(ⅱ)At a certain annealing temperature(850℃),the average grain size of the ferrite becomes larger and the volume fraction of the martensite is firstly increased and then decreased with extending the annealing time.When annealing time is 100 s,the martensite content is higher with a uniform distribution.With the extension of the annealing time,both the tensile strength and the yield strength increase first and then decrease,but the elongation shows little modification.(ⅲ)By changing the cooling rate,the volume fractions of the ferrite and martensite can be effectively controlled to obtain low yield strength and high tensile strength dual-phase steel.After annealing at 850℃,the tensile strength increases slightly with the change of the cooling rate,which is attributed to the ferrite,martensite and their interaction.With the increase of the cooling rate,the yield strength of the experimental steel increases firstly and then decreases.3.The effects of heating rate,annealing temperature and slow cooling temperature on the microstructural evolution of cold-rolled dual-phase steel were clarified.(i)At a certain annealing temperature(810℃),with increasing the heating rate,the volume fraction of the martensite islands is increased with a morphological change from long bar-like to equiaxed form.Meanwhile,their average size is decreased.When the heating rate is 70℃/s,the martensite islands become finer and more scattered,the size of which is about 1μm.Furthermore,with increasing the heating rate,the yield strength and tensile strength of the experimental steel are improved to different degrees.However,the drawing rate decreases slightly.When the heating rate is relatively high(e.g.70℃/s),the tensile strength of the experimental steel reaches 920 MPa,and the combination property is favorable,e.g.the product of strength and elongation reaches 16.4 GPa-%.(ⅱ)when the heating temperature increases from 780℃ to 810℃,the volume fraction of the martensite increases from 29.5%to 37%.Furthermore,the size of the martensite laths also increases,and the grain boundaries are transformed from clear to vague.With a further increase of the heating temperature,the variation tendency of the martensite phase fraction is attenuated.In addition,both the tensile strength and elongation increase firstly and then decrease,but the elongation shows little modification.(ⅲ)With the decrease of slow cooling temperature,the volume fraction of orientated ferrite increases,while the volume fraction of martensite decreases.Furthermore,the orientated ferrite has a low solid solution of carbon,which reduces the yield strength and promotes the ductility of ferrite.4.The effects of the substrate surface roughness,crystallographic texture and coating phase structure on the properties of alloyed coatings were specified.(ⅰ)A crystallographic texture analysis on the DC51D+ZF substrate surface indicates that the {001}-{101} orientation is more favorable for the formation of 8 phase,while the{11}-{113}-{313} orientation for the formation of Γ phase in the alloyed coating.The anti-chalking performance of the alloyed coating decreases with the increase of the Fe content and the thickness of the Γ phase layer,but the stripping performance is improved with the increase of the thickness of the Γ phase layer.The texture of the zinc layer itself and the forming ability of the substrate are also the reasons for the resistance to the powder.(ⅱ)The different anti-chalking properties of two SCG270 galvanized steel sheets with thicknesses of 0.7 mm and 1.6 mm were analyzed.Results show that:different Fe contents of the coating results in different phase structures,and different substrate roughness leads to different degrees of alloying.Roughness of the thick substrate is greater than that of the thin substrate.A higher roughness accelerates the degrees of alloying,and then gives rise to different Fe contents in the coating.The thickness of the alloyed coating increases as the substrate thickness increases.However,the thicker coating has a worse resistance to chalking.The unevenness of the coating thickness is also a factor that causes the coating to be different in chalking.更多还原